This invention pertains generally to the measurement of the internal pressure of an object such as the eye and more particularly to a system and method for analyzing signals produced by an applanating probe applied externally to the object.
Heretofore, applanating tonometers have been developed for use in determining intra-occular pressure without penetration of the eye. Such devices generally include a probe which is pressed against the cornea to flatten a portion thereof, and the force required to do the flattening is monitored to determine the pressure within the eye. The electrical waveforms or tonosignals produced by these probes are frequently transcribed and as transcribed are known as tonograms. An ideal tonogram is illustrated in FIG. 1.
From this figure it will be noted that the force on the probe increases initially, as indicated at 11, as the probe is brought into contact with the eye. While the cornea is being applanated a knee or breakpoint 13 occurs, and the force then decreases as indicated at 14. Thereafter, as the probe continues to press against the eye, the force increases to an overpressure as indicated at 16, with a notch or valley 17 between decrease 14 and increase 16. At the valley 17 a force balance exists at the surface of the eye, and the level of the waveform corresponds to the pressure which exists in the undisturbed eye before the tonometer is applied.
In practice, many tonosignals are encountered which do not have the ideal waveform shown in FIG. 1. Examples of such tonosignals are illustrated in FIGS. 2A-2F. In FIG. 2A, the waveform has a definite breakpoint 13, but no notch or valley 17. In FIG. 2B no overpressure 16 occurs, and this may or may not represent a good waveform, since the absence of overpressure can be due to premature pulling of the probe away from the eye. In FIG. 2C, the effect of hand tremor and/or motion of the probe at the surface of the eye is illustrated. While there are several breakpoints 18 in this waveform, only two (the third and fourth) exist for a sufficient time. Of these two, only one (the third one) is followed by an overpressure and is, therefore, likely to be acceptable. In FIG. 2D, the probe has been bounced at the surface of the eye, resulting in an unusable first waveform 19 and a second waveform 21 which is suspect. In FIG. 2E, the breakpoint 22 is only slight, indicating tight tonotips, improper probe angle or other factors. In FIG. 2F, no sufficient breakpoint exists, and this waveform should also be discarded.
In the past, tonosignals have been analyzed primarily by recording the waveforms on a graphic recorder to produce the tonograms for visual interpretation. This technique has a number of disadvantages, the most significant of which is that the interpretation can only be made by specially trained personnel at some time after the readings have been made and recorded.
There has also been at least one attempt to interpret tonometer signals electronically. U.S. Pat. No. 3,992,926 discloses a technique utilizing double differentiation to locate a point on the waveform to be read. While this approach may eliminate the need for a skilled operator, it is subject to other problems such as erroneous interpretation of some waveforms.